The Abelson (Abl) gene encodes a non-receptor protein tyrosine kinase (PTK). A mutant form of the gene is present in over 90% of patients with chronic myelogenous leukemia. The mutation is the result of the philadelphia chromosome rearrangement which leads to the production of a novel fusion protein, the bcr-able oncoprotein in haematopoetic cells. The presence of the oncoprotein provides progenitor cells with a growth advantage during the early chronic stages of the disease by unknown mechanisms. To learn more about the molecular mechanisms by which an Abl PTK can regulate cellular processes, we have taken advantage of the evolutionary conservation of the Abl gene and are using genetic strategies to study the function of the gene product in Drosophila melanogaster. Drosophila provides an economical experience mental system in which to study the effects of mutations in specific genes on normal development. One genetic strategy available in Drosophila is the recovery of the second- site mutations that alter the phenotype produced in the gene of interest, in our case the Abl gene. These second-site modifier mutations genetically tag genes strictly on the basis of functional interaction and do not require previous knowledge of the molecules involved. The approach will therefore be useful in identifying the unknown molecules in the Abl- regulated processes in Drosophila. We have used the approach to identify six genes in which mutations modify the phenotypes of mutations in the Drosophila Abl gene. We propose to examine the molecular mechanism by which the products of these genes interact by biochemical analysis of the protein; protein interactions. Protein:protein associations will also be used to identify additional proteins involved in the Abl-regulated processes. One of the genes identified in the genetic screens encodes a protein that is phosphorylated on tyrosine. Identification of the important substrates of the Abl PTK is a second goal of the proposed studies. The third goal of the proposed studies is the identification of six to ten additional genes in the Abl-regulated process using genetic backgrounds sensitized by mutations in the genes identified by previous screens. The fourth goal is to establish the phenotypic effects of perturbing the Abl-regulated process at a single cell level of resolution and to use phenotypic analysis to establish the functional epistatic relationships between genes involved in the process. This combination of biochemical, genetic, and morphological analyses will be used to test the hypothesis that the function of Abl in Drosophila is to mediate differential cell adhesion involved in several stages of development but especially important to the formation of axon connections in the central nervous system. The molecular mechanisms of this regulation may be relevant to human cells as it has been proposed that the bcr-abl oncoprotein alters specific cell adhesion properties of myeloid precursor cells in the etiology of chronic myelogenous leukemia.